Field of the Invention
The present invention relates to a current sensing circuit, and more particularly, to a current sensing circuit capable of compensating for degradation of an organic light emitting diode by stably sensing a current flowing on the organic light emitting diode, and to an organic light emitting diode display having the same.
Background of the Invention
Recently, various types of flat panel displays (FPDs) for reducing a large weight and large volume, and for addressing the disadvantages of a cathode ray tube, are being developed. Such flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an organic light emitting diode (OLED) display, etc.
Among the flat panel displays, the OLED display has advantages such as a rapid response speed, high light-emitting efficiency, high brightness and a large viewing angle, by using a spontaneous light emitting diode which emits light spontaneously.
The OLED display is provided with an organic light emitting diode (OLED), a spontaneous light emitting device, as shown in FIG. 1. The organic light emitting diode includes an organic compound layer (HIL, HTL, EML, ETL, EIL) formed between an anode electrode and a cathode electrode.
The organic compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL). Once a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer (HTL) and electrons passing through the electron transport layer (ETL) move to the emission layer (EML) to form excitons. As a result, the emission layer (EML) generates visible rays.
The OLED arranges pixels each having the aforementioned organic light emitting diode in the form of matrices, and controls brightness of pixels selected by a gate signal based on a gray scale level of a data signal, thereby displaying an image.
FIG. 2 is an equivalent circuit of a single pixel of an organic light emitting diode display according to the related art.
As shown in FIG. 2, each pixel of the organic light emitting diode display includes an organic light emitting diode (OLED), a gate line (GL) and a data line (DL) crossing each other, a switching TFT (ST), a driving TFT (DT) and a storage capacitor (Cst). Each of the switching TFT (ST) and the driving TFT (DT) are implemented as a P-type MOSFET.
The switching TFT (ST) is turned on in response to a gate signal provided from the gate line (GL), and conducts a current path between a source electrode and a drain electrode. The switching TFT (ST) applies a data signal provided through the data line (DL) to the driving TFT (DT) and the storage capacitor (Cst) during a turned-on period.
The driving TFT (DT) controls a current flowing on the OLED, based on a voltage difference (Vgs) between a gate electrode and a source electrode. The storage capacitor (Cst) maintains a gate potential of the driving TFT (DT) constantly for a single frame.
The OLED is connected between a drain electrode and a basis voltage (VSS) of the driving TFT (DT), with a structure shown in FIG. 1.
In the OLED display having pixels, a brightness difference between the pixels may occur due to an electric characteristic difference of the driving TFT (DT), or a degradation difference of the OLED. Especially, the degradation difference of the OLED occurs due to a different degradation speed of each pixel when the OLED display is operated for a long time. If the degradation difference of the OLED becomes severe, image sticking occurs. This may cause a picture quality to be deteriorated.